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Guide 1
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Step-by-step
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Insulation and
Continuity
Testing
10/05
Ring circuit
continuity
The ring circuit has become
standard for socket outlets, and,
as the name implies, is wired in
the form of a complete ring,
usually using twin and earth
cables. The two ends of the ring
are connected to the circuit
breaker or fuse at the consumer
unit, and we need to check that
the phase (often called the live),
the neutral and the earth
connections are continuous. We
will also test to ensure that there
are no cross connections (some
times called spiders).
1
2
Turn off the main isolator at the consumer unit
Remove the cover
The red WARNING VOLTAGE
PRESENT will flash with a warbling
warning tone.
There may be more than one
ring circuit, so identify the one you
wish to test. The board shown has ten
miniature circuit breakers (MCBs), four
of which are 32 A, one for the cooker
and the other three protecting ring
circuits. Switch off all the MCBs and
disconnect the cables from the one
whose circuit is to be tested. There will
be two red, two black and two
green/yellow-insulated conductors. If
the installation is a new one, the
conductors will be brown and blue
rather than red and black. Old installations may have bare earth conductors,
sometimes enclosed in green sleeving.
5
Measuring the phase (live) ring
continuity
8
Checking for “bridges” at socket
outlets
Now switch the DIT 400 to the
continuity measurement function by
pressing the “CONTINUITY” button.
Plug the crocodile clips into the leads,
connect them together and press the
“TEST” button. The display will show
the resistance of the leads, and pressing
the “NULL TEST LEADS” button will
ensure that this resistance value is
subtracted from all future continuity
readings. (Until switched off).
Next connect the clips to the two
disconnected red (or brown) phase
conductors. The “TEST” button will
then show the resistance of the
complete ring, and verify its continuity.
(Fig. 5).
7
Socket removed for testing
Next, go to each socket outlet,
remove it from its box (Fig. 7) and
test the resistance from phase to
neutral on each (Fig. 8).
As an alternative to removing
sockets, the Socket & See SS 130
Socket Adaptor can be used
(Fig. 7a).
Turn off the main isolator at the
consumer unit. Check that the
system has really been switched
off. With the tester SWITCHED
ON carefully put the probes on the
terminals at the live side of the
mains isolator.
7a
Using SS 130 Socket Test Adaptor
3
4
Testing the live side of the main isolator
Verifying that the consumer unit is
switched off
Next, put the probes on the
output side, where there should
be no indication. (Fig. 4).
The mains present LED should not
illuminate, and there will be no
warning tone.
6
Measuring the neutral ring continuity
Repeat this test with connections
to the two neutral conductors
(black or blue) (Fig. 6), with results
that should be about the same as
for the phase ring. A further test
on the earth ring should result in
values about 66% greater than
those for the phase and neutral
because the earth conductors
have smaller cross-sections.
Next we need to make sure that the ring is not
“bridged”, which happens if two spurs from
opposite points on the ring are joined up. First,
connect the phase (or live) conductor from one
end of the ring to the neutral from the other
end (red or brown to black or blue), and do the
same with the other two phase and neutral
conductors. The easiest way to do this is to join
the two pairs of conductors with a couple of
unplugged crocodile clips as shown in Fig 10. If
the wiring is carried out using twin with earth
cables, it will be obvious which conductors are
from opposite ends of the ring. In the case of
single-core conductors in conduit, great care
will be needed to ensure that the conductors
connected are from opposite ends of the ring.
9
The readings at all sockets should
be substantially similar except that
those wired on spurs will be a little
higher. If the readings are
different, for example if they
change steadily for sockets as you
test round the ring, there is
probably a bridge.
Now, repeat the process with
phase and earth conductors from
opposite ends of the ring
connected together (Fig. 10a).
Test from phase to earth at socket to
identify presence of “bridges”
Connect the two phase connectors to the two earth connectors
(Fig. 10a).
This time, the test at each socket
should be between phase and
earth (Fig. 9). This test also
confirms the correct polarity of
connection to each socket.
Continued Overleaf....
10
Connection of conductors for test using
disconnected crocodile clips (neutral)
Successful completion of these
tests shows that the ring circuit
has been properly wired and has
no bridges.
10a
Don’t forget to reconnect the
conductors to the circuit breaker
or the fuse exactly as they
were removed.
Connection of conductors for test using
disconnected crocodile clips (earth)
11
Lighting circuits are radial, not ring, circuits
and so do not require testing for bridges. It
is, however, most important to ensure that
all single-pole switches are connected in
the phase (live) conductor, rather than in
the neutral. One simple method here is to
make a temporary connection at the
consumer’s unit between the phase (red or
brown) and earth (green/yellow)
conductors. The DIT 400 is then set to the
continuity range and used to test between
the switch and earth at each position; a test
should also be made between the centre
contacts of ES lamp holders and earth.
This test ensures that the
insulation of the electrical system
is adequate. It involves connecting
a 500V direct current supply
between the points whose
insulation resistance is required.
The instrument reading is based
on the leakage current through
the insulation, the lower the
current, the higher the insulation
resistance. Tests are necessary
between phase (live) and neutral,
between phase and earth and
between neutral and earth.
Testing insultaion resistance
For all circuits the insulation resistance
should be at least 0.5 M Ω. However,
this is a very low value, and if the
reading is less than 2 M Ω there may
be need for further investigation. The
voltage used for testing can be lethal
to electronic equipment so all
vulnerable devices, such as fluorescent
fittings, passive infra-red detectors
(PIRs), touch switches, dimmer
switches, power controllers, delay
timers, electronic residual current
devices, etc. must be disconnected
before insulation testing takes place.
Lighting circuits
Insulation
resistance
The Socket & See DIT 400 will
automatically select the 500V
insulation range when you first
switch on so just connect your test
clips to the conductors to be tested.
Make sure that your fingers are
clear of the connections and then
press the “TEST” button, when the
insulation resistance will appear on
the display (Fig. 11).
(If you require other test voltages
such as 250V - for example testing
low voltage lighting circuits this can
be selected by pressing the “TEST
VOLTS” button - the display will
indicate the test voltage selected).
For this test, the resistance reading is
unimportant. As long as it is very low, the
polarity is correct. Don’t forget to remove
the temporary phase/earth connection
after the test!
Insulation tests are just as necessary on
lighting circuits as on others. When testing a lighting circuit, ensure that fluorescent fittings are disconnected, filament
lamps are removed from their holders and
that all switches are closed. The procedure
is the same as that given above for ring
circuits, but perhaps even more care is
needed to ensure that electronic devices
are disconnected to make sure that they
are not “killed” by the high test voltage.
Socket & See Limited
Rankine Road, Basingstoke RG24 8PP,
United Kingdom.
Tel +44 (0)1256 864100
e-mail: [email protected]
www.socketandsee.co.uk
10/05